Forskolin has been demonstrated to interact directly with the adenylyl cyclase catalytic subunit in diverse tissues. However, the ability of forskolin to bind and activate adenylyl cyclase is different depending upon the source of the tissue. Many different types of adenylyl cyclase have recently been cloned. The expression of the different types of adenylyl cyclase in a recombinant Sf9 baculovirus expression system provides an opportunity to study effects of forskolin on specific adenylyl cyclases. Previous work had established that bovine brain adenylyl cyclase binds forskolin with high affinity and has a nucleophilic group at the forskolin binding site. Recent work has established that the binding of forskolin to the Type I adenylyl cyclase is not modulated by the interaction of the Gs protein, in contrast to results with bovine brain. In contrast, the binding of forskolin to the Type II adenylyl cyclase is markedly increased by the interaction with the Gs protein alpha-subunit. Other modulators of adenylyl cyclase including calmodulin, G protein beta-gamma subunits, manganese, and adenosine P site inhibitors do not affect forskolin binding to Type I or Type II adenylyl cyclase. Ligand binding assays have also been developed to study the structure activity relationship for forskolin binding to the expressed adenylyl cyclases. These experiments will provide further information regarding the differences between adenylyl cyclase subtypes and will allow the development of subtype specific derivatives of forskolin.